Interpretive Summary: Examining how the rise in global atmospheric CO2 affects carbon released back to the atmosphere is key to knowing how much carbon can be stored in agricultural soils. Soil CO2 losses were monitored in a conventional (row crops rotated yearly, with tillage and winters fallow) and a conservation (row crops rotated yearly, with no-till, and winter cover crops) farming system which were exposed to two levels of atmospheric CO2 (ambient or elevated). Our results suggest that soil CO2 losses were higher under elevated atmospheric CO2 and with the conservation farming system, but these conditions can still increase soil carbon storage because they cause plants to grow bigger.

Technical Abstract:
Elevated atmospheric carbon dioxide (CO2) affects both the quantity and quality of plant tissues, which impacts the cycling and storage of carbon (C) within plant/soil systems and thus the rate of CO2 release back to the atmosphere. Research to accurately quantify the effects of elevated CO2 and associated feedbacks on soil CO2 efflux is needed in order to predict the potential of terrestrial ecosystems to sequester C. Soil CO2 efflux was monitored in a long-term study comparing row crops managed as either a conventional or a conservation tillage system and exposed to either ambient or elevated levels of atmospheric CO2. In the conventional system, grain sorghum and soybean were rotated each year using conventional tillage practices and winter fallow. The conservation system also uses a grain sorghum-soybean rotation, with three winter cover crops: wheat, crimson clover, and sunn hemp which were also rotated. All crops in the conservation system were grown using "no-till" practices. Plants were exposed to either ambient (370 ppm) or elevated (725 ppm) levels of atmospheric CO2 using open top field chambers. Soil CO2 efflux, over a full two-year cropping cycle, was increased by both elevated atmospheric CO2 and by conservation management. The higher efflux in both cases was due primarily to increased biomass inputs from these treatments. Despite the higher soil CO2 efflux, soil C was increased by both conservation management and elevated CO2.